Moon behaving badly

CELESTIAL MECHANICS is the grand old lady of science - reliable, precise, unfaltering. Ever since Newton drafted his laws of gravity and mechanics, scientists have used them to predict conjunctions and eclipses, to discover new planets on the basis of tiny gravitational disturbances, and to send spacecraft to the ends of the Solar System. It's all so predictable that we know where the planets and their moons will be thousands of years from now.

But two tiny satellites of Saturn seem intent on being the naughty children in the class. Between 1981 and 1995, these two moons changed speed, apparently defying the laws of gravity. Last summer, they misbehaved again, and no one knows why. Could this mean there's something about celestial mechanics that we're missing?

The two satellites were discovered on photographs snapped by NASA's Voyager 1 spacecraft when it flew past Saturn more than 20 years ago. Called Prometheus and Pandora, they orbit in the outer suburbs of Saturn's spectacular ring system, speeding around the planet in only 15 hours.

That brief fly-by didn't give astronomers enough information to establish the moons' orbits, but they didn't have long to wait before Voyager 2 took another set of photos in August 1981. From the movement of Prometheus and Pandora in the nine months between the two observations, astronomers calculated their orbits. From this, they were able to predict the moons' positions for any time in the future. Or so they assumed.

Nothing happened for almost 14 years. No new observations, no checks on the predictions. After all, why would anyone doubt the inevitable progress of celestial mechanics? Then, in the spring of 1995, a team led by Amanda Bosh of Lowell Observatory in Flagstaff, Arizona, took a look at Saturn and its tiny moons with the Hubble Space Telescope during a brief period in which the rings were seen almost edge-on. Pandora turned up exactly where it was supposed to be, but Prometheus was lagging 19° behind its predicted position. The satellite had apparently slowed down since the early 1980s, implying that it had drifted slightly away from the planet.

The astronomers were shocked. Prometheus and Pandora are quite small as moons go, but even so each is comparable in size to Northern Ireland or Connecticut, and must weigh many trillions of tonnes. So what could have pushed one of them tens of thousands of kilometres out of place?

One possibility is that Prometheus had been hit by a small asteroid or comet, but that's extremely unlikely. Realistically, the only force strong enough to have had such an effect is gravity. But the gravity of what? Theorists thought they had taken everything into account, but now they had to dream up some new explanations for the lag in Prometheus's motion. One was that there is some undiscovered moon in the same orbit as Prometheus, influencing its orbit. But to have the observed effect, this extra satellite would have to be fairly big, so the Voyagers should have seen it.

A more promising explanation came from Carl Murray of Queen Mary, University of London: he suggested that Prometheus was interacting with one of Saturn's rings. The moon orbits just inside the F-ring, a thin stream of debris that circles Saturn a little way outside the main ring system (see Diagram). Although Prometheus is usually a few hundred kilometres away from the ring, too far for gravity to have much effect, every 19 years or so there is a period when its orbit takes the moonlet almost into the ring during each revolution. Might there be enough mass in the F-ring to slightly change Prometheus's orbit?

The only way to test this idea was to keep a close eye on the misbehaving moon, and Richard French of Wellesley College, Massachusetts, again used the Hubble telescope to keep watch. In 1997 graduate student Colleen McGhee, based at Cornell University in Ithaca, New York, analysed the Hubble observations in the hope of finally tying Prometheus down. But instead of solving the mystery she discovered another one: Pandora was misbehaving too. Every 633 days, it speeded up, drifting away from its average position by almost a degree, before slowing down again and drifting back.

Although this was a surprise, it wasn't hard to come up with an explanation. Pandora's orbital period of 15 hours is almost exactly two-thirds that of Mimas, a much larger moon of Saturn. So the two moons are nearly in an "orbital resonance": after three orbits of Pandora, Mimas has gone around Saturn twice, and they're back in roughly the same relative position. This means that slight gravitational tugs between the two moons won't cancel each other out over time, as they would normally. Instead they are likely to add up to produce a relatively large effect - Pandora's oscillation.

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